This wasn't just another 1-in-500-years event happening, a freak occurrence, a one-off event. Rather, experts see it as the new normal across the Northeast, the latest in a series of calamitous weather events occurring because of, or amplified by, climate change.

From valleys staggered by Irene, to coasts battered by Superstorm Sandy, the 24-hour outbursts of rain and snow, or "extreme precipitation," has increased by 74% in the past six decades there, according to January's draft of the federal National Climate Assessment report.

Such storms have become the signature of climate change across the Northeast, afflicting older cities and towns built at a time of more modest rainfall. This heavy flooding is undermining aging bridges, eroding roads and overwhelming drainage systems.

USA TODAY traveled to the birch- and maple-dappled hills of Vermont as the third stop in a year-long series to explore places where climate change is already changing lives.

"I grew up here. I live here now," Hammond says, standing on the gravel road leading to the covered bridge's crossing. "The bridge has always been here."

That bridge, and hundreds more like it, were damaged in the storm, along with dams, roads, houses and most everything in sight. Three people died and more than $700 million in damage was sustained in Vermont alone. Before trailing off in Canada, Irene had killed 49 people and caused more than $10 billion in damages in 2011, according to the National Oceanic and Atmospheric Administration (NOAA). That was only a warm-up for last year's Superstorm Sandy, which delivered even more crippling blows, killing more than 130 people nationwide and causing more than $75 billion in damages.

Hammer blows such as these storms are consistent with climate change's expected effects on the Northeast, says climatologist Cameron Wake of the University of New Hampshire in Durham.

"It's not just those two storms," he says. "We are suddenly seeing many extreme precipitation events, ones that have led to several 100-year floods across the region just in the last 15 years."

The soggy misfortune of these extreme events has reached well beyond the Northeast. From the Upper Midwest, suffering from recent flooding, to the desert Southwest, where flash floods are more likely even amid sparser rain overall, high-intensity storms are more frequent. But nowhere is the increase as pronounced as in the Northeast, where already-moist air means more extreme, "extreme precipitation."

The more intense rainfall is a direct impact of warmer air temperatures tied to climate change juicing the weather cycle, according to the federal assessment report. Average temperatures in the region — the home of some 64 million people stretching from Washington, D.C., to Portland, Maine — have increased 2 degrees Fahrenheit in the past century, with most of the increase coming in the past three decades. The increase largely results from heat-trapping carbon dioxide and other greenhouse gases emitted into the air by burning coal, oil and other fossil fuels, boosting temperatures to spur increases seen worldwide in this century.

STORMS CHANGE 'EVERYTHING'

Amid trees just starting to bud with spring, Vermont's State Road 121 follows the twisting course of the Saxtons River. Along the way, Timothy Cullenen, who was municipal manager of nearby Rockingham, Vt., during Irene, points out wrecked homes and condemned, covered bridges with sidewalls beaten out and replaced.

"We had roads washed out, people trapped in homes," Cullenen says, recounting how firefighters had to boat people out of homes inundated by a river suddenly stuffed with battering-ram-size trees and hissing propane tanks with broken valves. The frequency of these storms "changes everything for us, from what size culverts we build, to where people can live," Cullenen says.

The Northeast averages about 55 inches of rain and snow annually, according to NOAA records. That's 10% more than just over a century ago.

What's causing the additional rain? It's simple. Warmer air causes more evaporation from streams, lakes and seas. Warmer air also holds more moisture. So, when it falls, it really unloads — thus, more extreme storms.

"Increased extreme precipitation in the Northeast is one of the clearest signals of climate change that we can see nationwide," says climate expert Donald Wuebbles of the University of Illinois in Urbana-Champaign. "It's not just more rain, but more rain falling in buckets over long periods of time."

In the hills of the Northeast, heavy rains are funneled into valley streams and rivers, carrying timber and sand downstream. "Bridges become dams when the water rises and this stuff slams into them," says FEMA historic preservation expert Peter Thomas.

In Irene, ground already saturated with rainwater couldn't handle the downpour, making flooding more likely. (Rainfall totaled more than 25 inches for much of the Northeast in August and September 2011, compared with about 8 inches normally.)

"Streams and rivers scoured out by floods just become race tracks for the next flood," Thomas says. "They won't have as much time to naturally restore themselves if climate change is making floods more common."

'OH MY GOD': BRIDGE CRUMBLES

"I never had been able to see the river from my window in August before," Hammond says of the day the bridge washed out. All morning, she and her neighbors had walked down to the bridge to see the river rise. Recording a video of the flooding, she captured the terrible creaking of the century-old structure and sight of it unrolling off its abutments into the water below. The wood tumbled in the river for a half-mile.

Hammond's video and heartfelt, "Oh, my God," response to the bridge's disappearance went viral on YouTube, seen by more than a half-million people and shown widely on television. "I had no idea it had gone viral until my sister told me days later, after we got power back," Hammond says.

The bridge had been in place since 1870. Now its wreckage sits, three rows of timber in Rockingham's winter gravel dump, joined by stumps, trees and junk pulled from the river by the town at a cost of $450,000.

"We used to have an emergency fund. And then we had an emergency. Now it is gone," Cullenen says. "But we still might have another emergency."

After record rainfall from Hurricane Irene, the Lower Bartonsville Covered Bridge is swept away. Scientists say climate change is causing more precipitation.

CITIES STRUGGLE TO ADAPT

Towns and cities across the Northeast are built for the wrong century — the last one — says civil engineer Paul Kirshen of Climate Solutions New England, a colleague of Wake, the climatologist. "We've seen the 'once in a century' storm's water level increase," he says. Where a storm with a 1% chance of happening every year used to dump 6 inches of water (162,924 gallons per acre), that same storm now delivers 8 inches (217,232 gallons per acre), according to the U.S. Geological Survey.

That means street drains aren't big enough and wastewater facilities can't handle floods. Culverts, the large pipes that carry water under roads, are increasingly being washed out across the region, according to a 2009 Journal of Applied Meteorology and Climatology study. Expensive sewer systems built for last century's specifications are too small to handle the extra water. More cars and more people mean more pavement, which dumps water on towns built in the horse-and-buggy era, instead of letting it soak in.

For cities such as Philadelphia and Exeter, N.H., one solution has been to try "low-impact" development: planting more greenery in a bid to soak up rainwater before it hits the drains. Another change has been "flexible" development, in which reservoirs and seawalls are designed so that additions can be made, if needed. "That's a total change in engineering," Kirshen says. "In the past, we built to specifications for now, and walked away."

A longer-term solution, Wake points out, is "putting less greenhouse gas in the air."

Depending on future emissions, average yearly temperatures will increase anywhere from 3 to 10 degrees by the 2080s, according to the draft federal National Climate Assessment report.

The series looks at different regions of the country.(Photo: USA TODAY)

NEW ENGLAND'S ESSENCE

Standing on the gravel road leading to her Vermont hamlet, Hammond looks at the rebuilt Lower Bartonsville Covered Bridge, its spruce sides gleaming blond in the sunlight, the sawmill scent of fresh-cut wood hanging in the air. The new bridge, built to historical standards, now exceeds its original length by 17 feet to stretch 168 feet across the Williams River. It rests on concrete abutments with pilings driven 40 feet into the ground — not the stone causeways that were undermined in the 2011 flood.

"A covered bridge is part of what New England is, and it connected us to the world," Hammond says. "Maybe you have to live here to understand it. It really was quite a blow to see it pulled away."

The new bridge is designed to last 75 years, taking it almost to the end of the century, when it will face a very different New England.

"Unless something happens again," Hammond says, before walking back to her home.

Screen shots from a video show the Lower Bartonsville Covered Bridge collapsing into the Williams River near Rockingham, Vt., on Aug. 28, 2011, as Tropical Storm Irene whipped through the state. "I grew up here," says Susan Hammond, who shot the video. "The bridge has always been here." Susan Hammond

Susan Hammond stands in front of the rebuilt Bartonsville Bridge near her home in Rockingham, Vt. Events of "extreme precipitation" have increased by 74% in the past six decades in the Northeast. Geoff Forester for USA TODAY

Power lines are down along a washed-out section of Route 4 in Mendon, Vt., on Aug. 29, 2011, the day after Tropical Storm Irene dumped heavy rains across the region. File photo by Vyto Starinskas, The Rutland Herald, via AP

Remains of the Lower Bartonsville Covered Bridge are pictured in a field along the Williams River, downstream from its original location. Tropical Storm Irene washed out the bridge in August 2011. File photo by Jim Cole, AP